1. Field of the Invention
The present invention relates to a power transistor switch for preventing breakdown at the time of turning off a power transistor switch which is used at a relatively high voltage such as in a chopper or in an inverter, etc.
2. Description of the Prior Art
Heretofore, the conventional type power transistor comprises a transistor 3 connected in series to the path connecting a power source 1 to a load 2 as shown in FIG. 1(a).
If the closed loop comprises only resistance, the Lissajous' diagram of the collector voltage V.sub.CE and the collector current I.sub.C for ON-OFF operation of the transistor 3 is shown as line A of FIG. 1(b). However, the closed loop comprises a wiring inductance and a load inductance. Accordingly, the resurge of the line A cannot be considered, especially in a power switch.
In usual applications for which a load inductance is not neglected, it has been customary to combine a diode 4 for commutating the load current during the OFF state of the transistor. In this case, the Lissajous' diagram is shown by the curve B of FIG. 1(b). An overshoot of the reapplied voltage is generated after the rise of the reapplied voltage while turning off the transistor with the wiring inductance.
A surge absorber 100 consisting of a capacitor 6 and a series resistor 5 is connected in parallel to the transistor for controlling the overshoot voltage. However, the Lissajous' diagram for a switch having a conventional surge absorber for controlling the overshoot is substantially the same as the resurge B of FIG. 1(b).
In order to improve the Lissajous' diagram, it is usually necessary to change the constant of the surge absorber by more than one figure. It is especially necessary to drastically decrease the resistance of the resistor 5.
The resistance should be decreased as follows. The resistance R.sub.5 of the surge absorber required for providing the I.sub.C -V.sub.CE trace from the ON state to the OFF state as the line A', FIG. 1(c), of a pure resistive load is given by the equation: EQU R.sub.5 .apprxeq.E/I.sub.L
wherein E designates a power voltage; and I.sub.L designates a load current.
On the other hand, when the switching is from the OFF state to the ON state, the current for discharge through the resistor 5 and the capacitor 6 is added. Accordingly, the I.sub.C -V.sub.CE trace is given as the curve C of FIG. 1(c) wherein the collector current peak value I.sub.CP is EQU I.sub.L +(V.sub.C /R.sub.5).apprxeq.2I.sub.L.
Incidentally, the transistor causes the sudden fall of current amplification and the rise of collector voltage by passing current over the rated collector current. Accordingly, the switch is not used in practice with the Lissajous' diagram condition of FIG. 1(c). Because of this reason, the surge absorber 100 having only the reapplied voltage overshoot control effect shown in the Lissajous' diagram condition of curve B of FIG. 1(b) has been used.
When a transistor is used as a power switch, the transistor is easily broken by a secondary breakdown such as a local thermal break phenomena, e.g.. a local temperature rise, a local breakdown for a short period, a local current concentration, a local thermal loss, etc., (hereinafter referred to as a secondary breakdown). In the conventional circuit, it has been necessary to derate (decreasing practical current and voltage to 1/2-1/4) the maximum rated collector current, the maximum rated collector-emitter withstand voltage V.sub.CEO (open circuit between base and emitter) and the collector-base withstand voltage V.sub.CBO (short circuit between base and emitter and substantially the same with the collector-emitter withstand voltage V.sub.CES at certain inverse bias).